Cathode coating



April 1 1958 W.'ROEBLING I 2,831,137

CATHODE comm;

Filed Feb. 6, 1956 FIG.1 FIG.2

I VE N TOR WE R ROEEJNG BY MUM ATTORNEY United rates Patent 9 CATHODE COATING Werner Roebling, Berlin-Charlottenbnrg, glermany, as-

signor to Patent-Treuhand-Gesellschaft fur Elektrischc Gliihlampen m. b. H., Munich, Germany Application February 6, 1956, Serial No. 563,754 Claims priority, application Germany February 23, 1955 11 Claims. (Cl. 313-109) This invention relates to low-pressure electric discharge devices filled with gas and/or vapor and, more part cularly, to fluorescent lamps and cathode emission coatings therefor.

Fluorescent lamps are usually provided with incandescent filamentary electrodes which carry a mixture of alkaline-earth oxides as electron-emission material. In such materials, the emission of electrons is effected principally by barium oxide which normally has a small stoichiometric excess of metallic barium as is well known. The other alkaline-earth oxides which may be included, e. g., strontium and calcium oxides, are eifective in retarding the evaporation of the barium oxide or of the metallic barium. It is well known that barium oxide has the lowest electronic work function as compared to the other alkaline-earth oxides, but it also has the highest vapor pressure so that it is normally preferably not to use an emission material consisting solely of barium oxide since the life is decreased, except in special applications.

It is the general object of this invention to provide animproved fluorescent lamp having an improved cathode electron emissive coating.

It is another object to provide a cathode coating which will lengthen the life of the lamp.

It is yet another object to provide a cathode coating which inhibits the tendency toward socalled end bandmg.

The aforesaid objects of the invention, and other objects which will become apparent as the description proceeds, are achieved by incorporating into the emission material from about 0.05% to 1.5% of finely divided sodium and/or potassium, that is, an alkali addition, the electronic work function of which is greater and the ion radius smaller than the corresponding values for the alkaline-earth materials influencing the emission.

For a better understanding of the invention, reference should be had to the accompanying drawing wherein:

Fig. 1 illustrates an elevational view of a fluorescent lamp;

Fig. 2 illustrates an axial view taken through an electrode of the lamp illustrated in Fig. 1.

It has been suggested to add a small quantity of water glass as a binder for the emission material and for increasing the adherability for the coil. The alkali contained therein serves only as a binder and the silica content is deleterious. Also, emission materials have been cataphoretically applied in which case salts of metals of the first to fourth group of the periodic table have been added to promote deposition, which metals have no effect on the emission. Further, when alkaline-earth carbonates have been prepared for emission materials, careful attention has been paid that the alkaline earth carbonates were completely free from the alkali salts.

In the manufacture of electrodes, the emission material is normally applied to a refractory metal coil, such as timgsten, by suspending the alkaline-earth carbonates and a binder, such as nitrocellulose, in an organic solvent,

such procedures being well known and standard in the art. The cathode coil is dipped into the suspended emission material which fills the turns between the coils. If a coiled-coil type is used, only the turns between the minor coils are filled with emission material. The coated electrodes are then mounted in the lamp and treated by heating to thedecornposition temperatures of the coated alkalineearth carbonates to convert the carbonates to oxides, as is well known.

In accordance with this invention, very good results have been obtained by incorporating about 0.31% of potassium carbonate and 0.14% by weight of sodium carbonate with the barium carbonate be reduced to the oxide. The carbonates of calcium and strontium, as used in the mixture may have only traces of alkali carbonates. Of course, the alkali addition could also be added to the strontium and/or calcium carbonates, if desired.

Whether the alkaline-earth carbonates are manufactured separately or are co-precipitated, the eifect is the same, that is, the life of the lamp is increased and the end blackening decreased. It should be noted that the ion radius of potassium, which is 1.33 A. U., lies between the ion radius of barium, which is 1.43 A. U., and that of strontium, which is 1.29 A. U. The ion radius of sodium, which is 0.98 A. U., is close to that of calcium, which is 1.06 A. U. Experience has proved that foreign ions may easily diffuse into a host lattice and adhere to it if the ion radius of the foreign ion does not differ from that of the host lattice ions by more than 10%. Therefore, it is probable that in the extremely fine distribution of the alkalies required for the desired effect, these alkalies are placedin an atomic state in the host lattice.

It has been proposed to add cesium to the alkalineearth compound, but these additions have been without any beneficial result. this invention, since cesium with an ion radius of 1.65 A. U. will not difluse into the host lattice of the alkaline earth oxides and will, therefore, not adhere at higher temperatures.

The efiect of traces of foreign admixtures upon electrophysical qualities, as for instance electronic work function and conductivity of semi-conductors, has been well known for a long time. It may be possible that a P-conductivity occurs in addition to the electronand ion-conductivity because of the alkali ions in the host lattice, thereby decreasing the percentage of ion conductivity. If such be the case, the holes in the valency band of the alkali-containing mixed crystals probably move toward the cathode, that is, toward the tungsten metallic base material, instead of the oxygen ions. Thus an increased life would result because of less electrolytic decomposition of emission oxides and a decrease in the speed with which oxidization of the tungsten coil takes place. Of course, it is well known that during the life of oxidecoated electrodes, barium tungstates formed by interaction with the base metal have a deleterious effect on the emission. The foregoing theory has yet to be proved, but it offers one theoretical explanation for the improvement realized by this invention.

In order to obtain the extremely fine, and probably atomic, distribution required for the desired effect of the alkalies, it is possible but not very favourable to admix mechanically sodium carbonate and/or potassium carbonate with the alkaline-earth carbonates. It is preferred, however, to obtain a very even distribution by first manufacturing alkaline-earth tartrate from soluble alkaline-earth salts and Rochelle salt (sodium-potassium tartrate). 1 i

The alkaline earth'tartrate prepared in the aforesaid manner is then liberated from excessive outward-adher- Patented Apr. 15, 1958.

This is understandable in view of i ing sodium-potassium tartrate by washing it whereby sufiicient quantities of sodium-potassium tartrate are still enclosed or incorporated by the insoluble alkaline-earth tartrate crystals. The alkaline-earth tartrate is then con verted into alkaline-earth carbonate by heating in an oxygen atmosphere at about 900 C. which heating also causes the alkali metals to be incorporated into the alkaline-earth compounds. It is pre erred to precipitate the alkaline-earth compounds separately, admixing same later. Not only does this simplify placing the desired amount of alkali material into the mixture, but also it is desirable that the strontium carbonate obtained from heating the' tartrate to 900 C. be reheated once more at 500 C. in a carbon dioxide atmosphere, in order to convert completely this compound to the carbonate. After the first heating, the strontium compound is a base, which has a deleterious effect on the binder and the second heating eliminates this Following is a specific example of the foregoing procedure:

Barium nitrate, 6400 cc. of 0.705 molar solution;

Rochelle salts, 2800 cc. of 1.77 molar solution;

Heat foregoing to 90 C. to precipitate barium tartrate; wash precipitate five times with 3000 cc. water to remove excessive and outwardly adhering alkali; heat washed precipitate to 900 C. for 1% hours in an oxygen atmosphere to convert to carbonates.

Strontium nitrate, 6200 cc. of 0.645 molar solution;

Rochelle salts, 2700 cc. of 1.63 molar solution;

Heat to 90 C. to precipitate strontium tartrate; wash precipitate five times with 3000 cc. of water to remove excessive alkali; fire at 900 C. for 1 /2 hours in an oxygen atmosphere and refire at 500 C. for 1%. hours in a C atmosphere to convert to carbonates.

Carbonates prepared in accordance with the foregoing specific examples will contain alkali carbonates as follows:

The percentages by weight of potassium and sodium with respect to the total barium in the emission material are then 0.25 for potassium and 0.09 for sodium.

The calcium carbonate, if such is to be used in the mixture, is preferably precipitated without the alkali addition, by methods which are well known. For example, calcium carbonate may be precipitated alkali free with ammonium carbonate and calcium nitrate.

As a specific example, barium carbonate and strontium carbonate, as prepared above, may be mixed in proportions of 1:1 by weight. Calcium carbonate may be substituted fully or partly for the strontium carbonate, if desired. As a further specific example, the barium, strontium and calcium carbonates prepared as above, may be admixed in the proportions by weight of 5:3:2 respectively.

Very good results have also been obtained in preparing the emission coating materials of this invention by adding a solution of alkali carbonates to the barium and strontium tartrates which have been precipitated free from alkali, for example. by precipitating the alkalineearth carbonates from ammonium tartrate and alkalinecarth water soluble salts. The admixed tartrates and alkali carbonates are then dried and heated to convert to carbonates and the alkalies will be very finely and evenly distributed throughout the alkaline-earth carbonates. In this manner, the percentages of alkali in the emission material may be very carefully controlled. Good results will be obtained where the percent by weight of potassium with respect to the total barium is from 0.04%

to 0.81% and the percent by weight of sodium with respect to the total barium is from 0.031% to 0.62%.

In test results using the prior art, alkali-free emission material coated cathodes as controls, lamps embodying the emission materials of this invention have displayed a increase in life. Also the first appearance of end blackening averaged about 1200 hours later in life than the control lamps.

With reference to the drawing, in Fig. 1 is shown a low-pressure gaseous discharge fluorescent lamp comprising a vitreous light-transmitting tubular envelope 1, having a luminescent material coated on the interior surface thereof, for example, the well-known zinc beryllium silicate. Sealed through the ends of the envelope are lead conductors which support electrodes 2 between their inwardly extending extremities. The electrodes 2 consist of a refractory metal, such as tungsten, coiled to receive the emission material of this invention between the coils. 0t course, the electrodes may also take the form of a coiled-coil, as is customary in many designs, in which case the emission material normally is coated between the turns of the minor coil. The envelope contains a small charge of mercury, as is customary and a small filling of inert, ionizable gas to facilitate starting, as is customary, a filling of argon at a pressure of 4 mm. mercury, for example, being suitable. Fig. 2 represents a section of the electrode 2 showing the emission material 3 of this invention held between the turns of the electrode coil.

It will be recognized that the objects of the invention have been achieved by the provision of an improved cathode coating for a fluorescent lamp cathode, which will increase the life of the lamp and decrease end blackening.

While in accordance with the patent statues, one bestknown embodiment has been illustrated and described in detail, it is to be particularly understood that the invention is not limited thereto or thereby.

I claim:

1. An electromemissive coating material for electric gaseous discharge device electrodes, said material always containing barium oxide and comprising at least one oxide of an alkaline-earth metal and a total of from about 0.05% to 1.5% by weight of emission material of at least one alkali metal having a work function greater than barium and an ion radius less than barium.

2. An electron-emissive coating material for electric gaseous discharge device electrodes, said material always containing barium oxide and comprising at least one oxide of an alkaline-earth metal and at least one of the group consisting of sodium and potassium, the percent by weight of sodium with respect to total barium expressed as metal being from 0.03l% to 0.62%, the percent by weight oi' potassium with respect to total barium expressed as metal being from 0.04% to 0.81%, and the total weight of said alkali metal being from about 0.05% to 1.5 by weight of said coating material.

3. An electron-emissive coating material for electric gaseous discharge device electrodes, said material comprising barium oxide and strontium oxide in the ratio of 1:1 and at least one of the group consisting of sodium and potassium, the percent by weight of sodium with respect to total barium expressed as metal being from 0.04% to 0.62%, the percent by weight of potassium with respect to total barium expressed as metal being from 0.04% to 0.81%, and the total weight of said alkali metal being from about 0.05% to 1.5% by weight of said coating material.

4. An electron-emissive coating material for electric gaseous discharge device electrodes, said material comprising barium oxide and strontium oxide and calcium oxide in the ratio of 5:312 and at least one of the group consisting of sodium and potassium, the percent by weight of sodium with respect to total barium expressed as metal being from 0.031% to 0.62%, the percent by weight of potassium with respect to total barium expressed as metal being from 0.04% to 0.81%, and the total weight of said 5 alkali metal being from about 0.05% to 1.5% by weight of said coating material.

5. An electron-emissive coating material for electric gaseous discharge device electrodes, said material always containing barium oxide and comprising at least one oxide of an alkaline earth metal and from 0.04% to 0.81% by weight of potassium with respect to total barium expressed as metal.

6. An electron-emissive coating material for electric gaseous discharge device electrodes, said material always containing barium oxide and comprising at least one oxide of an alkaline earth metal and about 0.25% by weight of potassium with respect to total barium expressed as metal.

7. An electron-emissive coating material for electric gaseous discharge device electrodes, said material always containing barium oxide and comprising at least one oxide of an alkaline earth metal and from 0.031% to 0.62% by weight of total barium of sodium with respect to total barium expressed as metal.

8. An electron-emissive coating material for electric gaseous discharge device electrodes, said material always containing barium oxide and comprising at least one oxide of an alkaline earth metal and about 0.09% by weight of sodium with respect to total barium expressed as metal.

9. A fluorescent lamp comprising a sealed, light-transmitting envelope, a coating of fluorescent material on the inner wall of said envelope, a quantity of mercury in said envelope, a filling of inert gas in said envelope, filamentary electrodes in said envelope and a coating material on said electrodes, said coating material always containing barium oxide and comprising at least one oxide of an alkaline-earth metal and at least one of the group consisting of sodium and potassium, the percent by weight of sodium with respect to total barium expressed as metal being from 0.031% to 0.62%, the percent by weight of potassium with respect to total barium expressed as metal being from 0.04% to 0.81% and the total weight of said alkali metal being from about 0.05% to 1.5% by weight of said coating material.

10. A fluorescent lamp comprising a sealed, light-transmitting envelope, a coating of fluorescent material on the inner wall of said envelope, a quantity of mercury in said envelope, a filling of inert gas in said envelope, filamentary electrodes in said envelope and a coating material on said electrodes, said coating material always containing barium oxide and comprising at least one oxide of an alkaline-earth metal and from 0.04% to 0.81% by weight of potassium with respect to barium expressed as metal.

11. A fluorescent lamp comprising a sealed, light-transmitting envelope, a coating of fluorescent material on the inner wall of said envelope, a quantity of mercury in said envelope, a filling of inert gas in said envelope, filamentary electrodes in said envelope and a coating material on said electrodes, said coating material always containing barium oxide and comprising at least one oxide of an alkaline-earth metal and from 0.031% to 0.62% by weight of sodium with respect to barium expressed as metal.

References Cited in the file of this patent UNITED STATES PATENTS 1,817,636 Meyer Aug. 4, 1931 2,201,167 Germeshausen May 27, 1940 2,530,394 Lowry Nov. 21, 1950 2,677,623 Delrieu May 4, 1954 2,714,681 Keiffer Aug. 2, 1955 

1. AN ELECTRON-EMISSIVE COATING MATERIAL FOR ELECTRIC GASEOUS DISCHARGE DEVICE ELECTRODES, SAID MATERIAL ALWAYS CONTAINING BARIUM OXIDE AND COMPRISING AT LEAST ONE OXIDE OF AN ALKALINE-EARTH METAL AND A TOTAL OF FROM ABOUT 0.05% TO 1.5% BY WEIGHT OF EMISSION MATERIAL OF AT LEAST ONE ALKALI METAL HAVING A WORK FUNCTION GREATER THAN BARIUM AND AN ION RADIUS LESS THAN BARIUM. 